Embodiments described herein relate to a diesel engine exhaust treatment system and method, and more particularly, to an exhaust treatment system and method which utilize a platinum group metal trapping device to prevent contamination of SCR catalysts.
Diesel engine exhaust treatment systems are known for use in converting gaseous emissions such as nitrogen oxides (NOx) to environmentally acceptable compounds. Such systems typically include a diesel oxidation catalyst (DOC), a selective catalytic reduction catalyst (SCR), and a diesel particulate filter (DPF).
Diesel oxidation catalysts are placed in the exhaust gas stream of a diesel engine and typically contain platinum group metals (PGM), base metals, or a combination thereof. These catalysts promote the conversion of CO and HC emissions to carbon dioxide and water.
Selective catalytic reduction catalysts (SCR) are used to convert NOx to N2 and typically comprise a base metal and utilize an ammonia reductant, typically in the form of aqueous urea, which is injected into the exhaust stream downstream from the diesel oxidation catalyst. After water vaporization and urea hydrolysis, the formed ammonia reacts with NOx in the exhaust gas stream on the SCR catalyst to form N2.
A diesel particulate filter (DPF) collects soot or particulate matter from engine exhaust. A precious metal catalyst selected from platinum group metals is typically coated on the DPF for the removal of CO, HC, and NH3 slip.
It has been observed that treatment systems which include diesel oxidation catalysts washcoated with platinum group metals may lose trace amounts of platinum or other platinum group metals under certain conditions, such as high temperature operation. The trace amounts of platinum group metals from the DOC sublime and then accumulate on the SCR catalyst positioned downstream from the DOC such that the function of the SCR catalyst is inhibited. This is due to the high activity of platinum group metals for ammonia oxidation which results in little or no ammonia being available for the SCR reaction. In addition, because platinum group metal catalysts generate NOx and N2O from ammonia, an increase in NOx levels could actually result when the SCR is inhibited due to platinum group metal poisoning. It has been discovered that platinum levels as low as 0.0005 wt % can have a significant impact on SCR performance. See, Jagner et al., “Detection, Origin, and Effect of Ultra-Low Platinum Contamination on Diesel-SCR Catalysts,” SAE International Paper No. 2008-01-2488, (2008).
Accordingly, there is a need in the art for a diesel engine exhaust system which prevents the deposition of platinum group metals on an SCR positioned downstream from a diesel oxidation catalyst, and to a system which efficiently achieves conversion of combustion components and removal of particulates in the exhaust gas.